The present invention relates to a technique for providing enhanced security for communication data in a data transfer system.
In recent years, portable devices such as mobile phones are used with highly confidential data such as a credit card number. Since a portable device is often carried around with the user, it is quite possible that the device may be lost and come into possession of a third person. With mobile phones, users frequently buy new handsets to replace old ones, and the used handsets are either collected or disposed of If confidential data is left undeleted in a collected or disposed handset, there is a high risk that the confidential data may be obtained by a third person. Therefore, security techniques for portable devices are becoming more and more important.
On the other hand, portable devices are powered by batteries, and it is therefore important that they consume as little power as possible. Each year, the amount of power consumed by a system for use in a portable device has been reduced, and various techniques have been proposed in the art. For example, “Bus-Invert Coding for Low Power I/O” (IEEE Transaction on VLSI Systems, Vol. 3, No. 1, 1995) discloses a method for reducing the amount of power consumed in a data transfer path between semiconductor devices. According to this method, if there are more mismatched bits, than matched bits, between data transferred at time T and data transferred at time T+1, the data transferred at T+1 is bit-inverted before being transferred. In this way, the number of switching transitions occurring on an I/O pad is reduced, and the power consumption can be reduced.
Problems to be Solved by the Invention
However, the above conventional method is for reducing the power consumption, and does not deal with security problems. FIG. 9 is a conceptual diagram illustrating a configuration of a data transfer system employing the conventional method. Referring to FIG. 9, a signal line 51 carries the transferred data, and a signal line 52 carries a 1-bit signal that indicates whether or not the transferred data has been inverted. If the signal lines 51 and 52 can be probed from outside, the transferred data can easily be observed.
As a technique for providing enhanced security for data transfer between devices, Japanese Laid-Open Patent Publication No. 08-32574 discloses a method for realizing enhanced security for data in a synchronous serial data transfer device. According to this method, the sync signal is encrypted in addition to encrypting the transferred data, thereby improving the security of the data at a relatively low cost.
However, it is assumed that the method is used with serial data transfer, and the use of the method with parallel data transfer will cause an increase in the circuit area. While there are other coding methods for parallel communications that use complicated encryption schemes, those methods require a separate encryption circuit. Moreover, since such an encryption circuit itself consumes some power, it is difficult to reduce the cost and the power consumption.